Fixed liveness bug for null array units

[Oofky]

Description

This PR fixes a small bug we found where null array units sometimes still show the default stroke color despite being dead/unreferenced. It is a very minor fix and follows the same logic as other objects such as regular array units, so it will not break anything.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Code quality improvements

How to test

Run the following code (or any code where the frame that references an array with a null unit is dead), and observe that the stroke representing the null unit is greyed out when appropriate:

{
    let a = list(2, null, null);
}
const c = 3;

Before:

Recording.2026-03-09.232820.mp4

After:

Recording.2026-03-09.234600.mp4

Checklist

• I have tested this code

[martin-henz]

Looks good!